In recent years, nitride-based semiconductors, typified by gallium nitride (GaN), and wide band gap semiconductors, such as silicon carbide (SiC), have been actively studied and developed as materials for semiconductor devices. Wide band gap semiconductors have a breakdown field greater than that of silicon (Si) semiconductors by an order of magnitude. When conventional Si semiconductors are used to obtain a power semiconductor device having a high breakdown voltage, the power semiconductor device needs to have a long drift layer in which electrons travel. However, when nitride-based semiconductors or SiC is used, an equal breakdown voltage can be achieved by a drift layer having a length which is about 1/10 compared to when Si semiconductors are used. When a current is passed through the semiconductor device, the drift layer becomes a resistor layer. Therefore, the on-resistance of the semiconductor device can be reduced by using a wide band gap semiconductor which has a greater breakdown field and therefore can provide a shorter drift layer. The on-resistance of a semiconductor device having a predetermined breakdown voltage is inversely proportional to the third power of the breakdown field if the semiconductor material has the same mobility and permeability.
Nitride semiconductors such as GaN and the like can be used together with aluminum nitride (AlN), indium nitride (InN) or the like to produce various compounds. Therefore, nitride semiconductors can be used to produce a heterojunction as with conventional arsenide-based semiconductor materials such as gallium arsenide (GaAs) and the like. In particular, in the heterojunction of nitride semiconductors, a high concentration of carriers are generated at the interface by spontaneous polarization or piezoelectric polarization even in the absence of doping with an impurity. As a result, nitride semiconductors can be used to achieve high power devices which are lateral devices in which a current is caused to flow in a direction parallel to the substrate, and which have a low on-resistance and which can flow a large current.
Moreover, by introducing into lateral devices a double gate structure in which a first gate electrode and a second gate electrode are provided between a first ohmic electrode and a second ohmic electrode, it is possible to achieve bidirectional switches in which a current flows bidirectionally and a high breakdown voltage is bidirectionally provided.
Bidirectional switches for use in matrix converters, drive circuits for plasma display panels (PDPs), and the like have been generally developed using a reverse blocking insulated gate bipolar transistor (IGBT). However, when the reverse blocking IGBT is applied to bidirectional switches, two reverse blocking IGBTs needs to be arranged in an antiparallel fashion. Because IGBTs intrinsically have a turn-on voltage across the PN junction, the on-resistance is large in a region in which a small current flows, resulting in a large power loss during switching.
In bidirectional switches having a double gate structure, as shown in FIG. 11(a), when a bias voltage is simultaneously applied to a first gate electrode G1 and a second gate electrode G2, a current can be caused to flow bidirectionally between a first ohmic electrode S1 and a second ohmic electrode S2 without a rising voltage. As shown in FIG. 11(b), when a bias voltage is applied to only one of the two gate electrodes, rectification is performed in which a current flows only in one direction. Therefore, a bidirectional switch which has a low power loss during switching can be achieved using a single chip (see, for example, Non-Patent Document 1).
In bidirectional switches having a double gate structure, a large gate width is required for a large current. As a technique of increasing the gate width, a chip layout has been studied in which a plurality of unit cells in which a first gate electrode and a second gate electrode are provided between a first ohmic electrode and a second ohmic electrode are arranged in parallel (see, for example, Patent Document 1). As a result, a limited area can be efficiently used to easily increase the gate width.